One-dimensional nanostructures, such as belts, rods, tubes and wires, have become the latest focus of intensive research with their own unique applications. One-dimensional-nanostructures are model systems to investigate the dependence of electrical and thermal transport or mechanical properties as a function of size reduction. In contrast with zero-dimensional, e.g., quantum dots, and two-dimensional nanostructures, e.g., GaAs/AlGaAs heterojunctions and superlattices, direct synthesis and growth of one-dimensional nanostructures has been relatively slow due to difficulties associated with controlling the chemical composition, dimensions, and morphology. Alternatively, various one-dimensional nanostructures have been fabricated using a number of advanced nanolithographic techniques, such as electron-beam (e-beam), focused-ion-beam (FIB) writing, and scanning probe.
Carbon nanotubes are one of the most important species of one-dimensional nanostructures. Carbon nanotubes are one of four unique crystalline structures for carbon, the other three being diamond, graphite, and fullerene. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall (single-walled nanotubes) or multiple walls (multi-walled nanotubes). These types of structures are obtained by rolling single layers of graphene sheets into cylinders forming a plurality of hexagons on the tubes' surface. The sheet is a close packed array of carbon atoms having no dangling bonds. Carbon nanotubes typically have a diameter on the order of a fraction of a nanometer to a few hundred nanometers. As used herein, a “carbon nanotube” is any elongated carbon structure.
Carbon nanotubes can function as either a conductor (metallic) or a semiconductor, according to the rolled shape (chirality) and the diameter of the helical tubes. With metallic nanotubes, a one-dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic nanotubes can be used as ideal interconnects.
Another class of one-dimensional nanostructures is nanowires. Nanowires of inorganic materials have been grown from metal (Ag, Au), elemental semiconductors (e.g., Si, and Ge), III-V semiconductors (e.g., GaAs, GaN, GaP, InAs, and InP), II-VI semiconductors (e.g., CdS, CdSe, ZnS, and ZnSe) and oxides (e.g., SiO2 and ZnO). Similar to carbon nanotubes, inorganic nanowires can be synthesized with various diameters and length, depending on the synthesis technique and/or desired application needs.
Both carbon nanotubes and inorganic nanowires have been demonstrated as field effect transistors (FETs) and other basic components in nanoscale electronic such as p-n junctions, bipolar junction transistors, inverters, etc. The motivation behind the development of such nanoscale components is that “bottom-up” approach to nanoelectronics has the potential to go beyond the limits of the traditional “top-down” manufacturing techniques. A carbon nanotube is also known to be useful for providing electron emission in a vacuum device, such as a field emission display. The use of a carbon nanotube as an electron emitter has reduced the cost of vacuum devices, including the cost of a field emission display. The reduction in cost of the field emission display has been obtained with the carbon nanotube replacing other electron emitters (e.g., a Spindt tip), which generally have higher fabrication costs as compared to a carbon nanotube based electron emitter.
One well known way of growing one-dimensional nanostructures is by CVD, however, this is a high temperature process that may prevent simple integration of carbon nanotubes with other device fabrication processes.
One known approach to manufacture nanowires is a top-down approach which uses e-beam lithography. However, this e-beam process is not desirable for mass production due its throughput limitations. Nanowire devices have also been fabricated by post synthesis assembly techniques, such as dispersion on an insulating substrate followed by patterning of electrodes on a few selected nanowires using lithography. Furthermore, nanowire synthesis methods typically, whether chemical vapor deposition or solution based, produce nanowires with a range of dimension and a range of properties. Conventional nanowire fabrication approaches include forming the nanowire using, for example, chemical vapor deposition (for crystalline semiconducting nanowires) or porous alumina membrane as a template (for metallic nanowires). Once the nanowires are fabricated, they are assembled on a substrate using either a random assembly approach or an ordered approach using micro fluidic channels for potential application.
Accordingly, it is desirable to provide a simple yet reliable technique to assemble one-dimensional nanostructures selectively in a desired pattern for device applications. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.